Fischer Tropsch (FT) synthesis involves the conversion of carbon monoxide and hydrogen to higher hydrocarbon products. In the case of Low Temperature Fischer Tropsch (LTFT) synthesis, wax is the penultimate product. Wax is converted by hydrocracking into shorter chains for use as high quality transportation fuels, mainly diesel fuel.
In the case of LTFT processes, the reactor is typically a Slurry Bubble Column Reactor (SBCR). Synthesis gas, a mixture of carbon monoxide and hydrogen, is bubbled through a column of liquid wherein catalyst particles are suspended in the SBCR. The catalyst suspended in the liquid column catalyses the conversion of the synthesis gas to form predominantly liquid higher hydrocarbons. These liquid hydrocarbons (wax product) are removed from the SBCR by a liquid-solid separation means, normally filtration. Filters can be placed internally within the SBCR or externally. The catalyst particle size and filter mesh size are normally carefully selected within a specific range to compliment each other to ensure that the catalyst is retained in the SBCR or can be circulated back to the SBCR in the case of externally placed filters. A further requirement is that the liquid product does not contain excessive catalyst.
Due to the extreme hydrodynamic forces within the SBCR the catalyst particles tend to undergo attrition. Attrition increases the number of fine particles (<25 microns) and reduces the average particle size. The presence of catalyst fines leads to separation difficulties, can prematurely block filters and result in catalyst breakthrough of the filters and enable catalyst to become entrained in the liquid flow. Further hydroprocessing of such particle containing higher hydrocarbons (liquid wax product) will result in premature deactivation, fouling and eventual blockage of such hydroprocessing catalysts.
As per the FT catalyst art, FT catalysts are typically supported on various refractory supports such as alumina, silica and titania. Group VIII refractory supported metals are used to catalyse the FT reaction, these include cobalt, iron, nickel and ruthenium. Promoters may be added to the catalyst and could include ruthenium, palladium or platinum, rhenium, lanthanum and zirconium.
Although hydrocracking is a well-established and widely practiced technology, the prior art relating to the clean up and removal of particulate from hydroprocessing feeds is all based on crude oil feeds and does not cater for FT derived feeds. FT derived feeds differ vastly from crude based feeds in that they essentially comprise of linear, paraffinic hydrocarbons, are free from sulphur, nitrogen, however, may contain traces of catalyst fines including cobalt and aluminium (alumina).
Prior art methods involve the filtering of feeds through various types of filter media. Particles down to about 1 micron can be removed, however, using large filter surfaces and with frequent replacement of filter media. The latter is undesirable for continuous processing since small pore filters are prone to irreversible plugging.
Prior art technologies have been found to be unsuitable for the removal of catalyst ultra fines and portions of soluble catalyst metals.